Abstract Irritable bowel syndrome (IBS) affects 10-20% of adults (women>men) worldwide exerting a tremendous economic, social, and emotional burden. Increasingly it is becoming clear that IBS is a heterogeneous condition both in terms of clinical presentation (diarrhea versus constipation) and pathophysiology. Patients with IBS report a number of co-morbid conditions and symptoms including poor sleep. On a day-to-day basis poor sleep the night before is a predictor of gastrointestinal (GI) symptoms as well as psychological distress the next day in IBS patients. In addition, evidence indicates that cortisol levels are higher during the night at least in a subgroup of patients with IBS. Yet, how sleep and visceral sensitivity and bowel symptoms are linked remains limited. The focus of the proposed study is on characterizing a metabolic pathway that may distinguish a subgroup of patients with IBS with comorbid poor sleep. Findings could result in a paradigm shift from bowel pattern plus abdominal pain/discomfort symptoms to one that encompasses co-morbid conditions such as poor sleep and biological markers. The study will utilize samples from 59 women (ages 18-45; follicular phase) previously studied in a sleep laboratory and in whom serial blood samples, polysomnography, adrenocorticotropic hormone, cortisol, and targeted genetic markers were obtained. Nine samples starting prior to sleep onset and continuing during the night will be assayed at the Northwest Metabolomics Research Center for metabolites that are in the tryptophan pathway. Previous research has demonstrated that several of these metabolites (melatonin, serotonin, kyneurinine) are linked with sleep, mood state, motility, pain sensitivity, and inflammation. Thus the aims of the study are to compare plasma metabolites gathered at 9 times before and during sleep to 1) describe and compare plasma tryptophan (TRY) metabolite patterns in women (18-45 yr. of age) with IBS (n=38) to HCs (n=21); 2) test the relationship of TRY metabolites with cortisol/ACTH ratio; 3) test the relationship of sleep quality (PSQI, PSG, sleep diary) with TRY metabolite patterns; and 4) explore the relationship of sleep quality indicators, cortisol and ACTH with additional metabolic pathways and targeted serotonin-related gene associations. Based on preliminary work we hypothesize that the melatonin/niacinamide ratio will be lower in IBS women compared to controls; cortisol will be negatively correlated with melatonin; self-report of sleep quality (diary, PSQI) and sleep efficiency will also be positively correlated with melatonin. Metabolomic fingerprinting (150 metabolites) combined with genetic polymorphisms in the rate-limiting tryptophan hydroxylase gene will allow us to further elucidate the relationship of poor sleep with a common condition and design optimal therapeutic approaches.